(1) Field of the Invention
The present invention generally relates to a wavelength-division-multiplex (WDM) transmission technique, and more particularly to a technique suitable for use in a WDM transmission system employing optical add-drop multiplexers for optical signals.
(2) Description of the Related Art
It is desirable in the backbone of a network to change communication capacity according to a span between nodes constituting the network. This is because the span between nodes is about 80 km and has a different demand for communication. It is also desirable to allocate multiple wavelengths to a span that has a great demand for communication, because a demand for communication varies with a time zone of a day.
Hence, an optical add-drop multiplexer (hereinafter referred to as an OADM node), capable of increasing or decreasing the number of wavelengths by dropping and adding optical signals, is employed in a WDM transmission system to meet an increase or decrease in traffic between WDM nodes.
A WDM transmission system employing such an OADM node is mainly utilized, for example, in a system with a terminal-opposed structure shown in FIG. 17, a system with a ring structure shown in FIG. 18, etc.
That is, in the system with a terminal-opposed structure shown in FIG. 17, at least one OADM node 103 is disposed on the optical transmission lines 107, 108 between WDM terminals 102, 105. At the OADM node 103, an optical signal of specific wavelength in a wavelength-division-multiplex (WDM) signal received from the WDM terminal (hereinafter referred to simply as the terminal) 102 or 105 is dropped to a network element (NE) 104 belonging to the OADM node 103. An optical signal from the NE 104 is added to the vacant wavelength of the WDM signal generated by dropping the specific wavelength. In this way, communication is performed between the NE 101 (or 106) belonging to the terminal 102 (or 105) and the NE 104 belonging to the OADM node 103.
In the WDM transmission system with a ring structure shown in FIG. 18, a plurality of OADM nodes 103a, 103b, 103c, and 103d are connected in ring form through optical transmission lines (optical fibers). The determination of how optical signals of different wavelengths are dropped and added at different OADM nodes 103a to 103d is made in advance. In this manner, communication is performed between the NEs 104 belonging to different OADM nodes 103a to 103d (hereinafter referred to simply as OADM nodes 103).
In FIG. 18, the solid line between the OADM nodes 103 represents a transmission line for each wavelength of a WDM signal which is transmitted through the above-described optical transmission line 107 (108). The above-described NEs 104 are equivalent to transmitters that are employed in a synchronous digital hierarchy (SDH)/synchronous optical network (SONET), an Ethernet of a gigabit level, etc.
The above-described add-drop OADM node 103 typically has a multiplexing-demultiplexing structure such as that shown in FIG. 19 (constructed by two pairs of wavelength multiplexing-demultiplexing terminals), or a WDM filter structure such as that shown in FIG. 20.
In the former, as shown in FIG. 19, a transmitting system of two-way communication (upper half portion in FIG. 19) includes a first optical amplifier 111, a second optical amplifier 117, a wavelength demultiplexer 112, optical drop couplers 113 corresponding to the number of wavelengths, optical switches 114 corresponding in number to the optical drop couplers 113, optical add couplers 115 corresponding in number to the optical switches 114, a wavelength multiplexer 116, and variable optical attenuators 120 corresponding in number to the optical add couplers 115. Similarly, a receiving system (lower half portion in FIG. 19) includes a first optical amplifier 111, a second optical amplifier 117, a wavelength demultiplexer 112, optical drop couplers 113 corresponding to the number of wavelengths, optical switches 114 corresponding in number to the optical drop couplers 113, optical add couplers 115 corresponding in number to the optical switches 114, a wavelength multiplexer 116, and variable optical attenuators 120 corresponding in number to the optical add couplers 115.
In addition, the above-described transmitting system and receiving system are provided with opto/electric (O/E) converters 121 for handling an optical supervisory channel (OSC), LSIs 122, electro/optical (E/O) converters 123, and spectrum analyzers 124 for a WDM signal, respectively.
In FIG. 19, each of the right and left parts of the OADM 103 with the optical switches 114 as the center is approximately equivalent to the construction of a single terminal that already exists. In addition, as the optical amplifiers, erbium-doped fiber amplifiers (EDFAs) are widely employed. In high-speed communication, a phenomenon (dispersion of wavelength) that the velocity of light varies little by little with wavelength will occur in optical fibers that are employed in the optical amplifiers 111, 117 and optical transmission line 107 (108). To compensate for this phenomenon, there are provided dispersion compensation fibers (DCFs) 118, 119.
In the OADM 103 with such a multiplexing-demultiplexing structure, a WDM signal received through the optical transmission line 107 (108) is first amplified to the required optical signal level by the optical amplifier 111 to compensate for losses which will occur due to the demultiplexing at the optical drop couplers 113. Then, the amplified WDM signal is demultiplexed into optical signals of different wavelengths (channel signals) by the optical demultiplexer 112. Next, an optical signal of a wavelength to be separated is dropped to the NE 104 by the corresponding optical drop coupler 113, and the optical signals of wavelengths other than the dropped wavelength are passed through the optical switches 114.
Note that the demultiplexed optical signals, including the optical signal of a dropped wavelength, are passed through the optical drop couplers 113. However, the optical signal of a dropped wavelength is not passed through to make a vacant wavelength for the addition of an optical signal at the subsequent stage. That is, the optical signal of a wavelength dropped at the self-node 103 is stopped by the corresponding optical switch 114 so that it is not sent to the subsequent stage.
Thus, in the OADM node 103, an optical signal with data transmitted from another NE 104 can be added by the corresponding optical add coupler 115 to a grid that has a vacant wavelength by dropping a selected wavelength. The added optical signal and the above-described optical signals of wavelengths other than the dropped wavelength are incorporated into a WDM signal by the optical multiplexer 116. The WDM signal is again amplified by the optical amplifier 117 to compensate for losses and is transmitted onto the downstream transmission line 107 (108). Note that the addition of an optical signal to a vacant wavelength may be performed at an arbitrary node of the following stage as well as the node 103 that dropped an optical signal of a selected wavelength.
Incidentally, the information about the wavelengths of a WDM signal that is transmitted through the OADM node 102 is being monitored by the spectrum analyzer 124. The wavelength information is fed back to the optical control system of the self-node 102. The information includes measured information such as an optical signal-to-noise ratio (OSNR), amplified spontaneous emission (ASE) light, etc. Based on the information, an optical transmission level, etc., are adjusted so that the state of an optical transmission signal becomes optimum. For instance, the level of an optical signal of a wavelength added is adjusted by adjusting the degree of attenuation of the variable optical attenuator 120.
In addition, according to optical power information obtained from the spectrum of a WDM signal for each node 103, preemphasis control is performed between nodes 103 to reduce a wavelength-dependent characteristic between nodes and ensure stable communication. The above-described monitor control information is transmitted by employing the above-described OSC. In WDM transmission systems, one or more wavelengths are allocated beforehand for the OSC. In each node 103, information transmitted through the OSC is converted into an electrical signal by the above-described O/E converter 121. The electrical signal is analyzed by the LSI 122. Information corresponding to the result of analysis is again converted into an optical signal of the original wavelength by the E/O converter 123 and is transmitted. In this way, the monitor control information can be transmitted through the OSC.
On the other hand, in the OADM node 103 with a WDM filter structure, as shown in FIG. 20, the above-described add-drop function (realized by the optical demultiplexer 112, optical drop couplers 113, optical add couplers 115, optical switches 114, and optical multiplexer 116 shown in FIG. 19) is realized by optical drop couplers 125 corresponding to the number of wavelengths, optical add couplers 126 corresponding to the number of wavelengths, a rejection filter 127, predetermined filters 128 corresponding in number to the optical drop couplers 125, and predetermined filters 129 corresponding in number to the optical add couplers 126.
In such a structure, a received WDM signal from the optical amplifier 111 is demultiplexed by the optical drop couplers 125. Only an optical signal of a wavelength to be dropped is selected by the predetermined filter 128 and is dropped to the NE 104. As in the aforementioned case, the optical signal of the dropped wavelength passed through the optical drop coupler 125 is rejected by the rejection filter 127 to make a vacant wavelength at the following stage.
Thus, an optical signal of the added wavelength equal to the dropped wavelength, and the optical signals of wavelengths other than the dropped wavelength, passed through the rejection filter 127, are incorporated into a WDM signal by the optical add couplers 126. Note that the operation other than the above-described adding-dropping function is the same as the operation described in FIG. 19.
As described above, the conventional WDM transmission systems can meet an increase or decrease in traffic between nodes by the OADM node 103 which has the function of adding and dropping an optical signal through the optical switches 114 or rejection filter 127.
In the existing WDM transmission systems, incidentally, transmission is performed without recognizing the content of information about an optical signal [signal format (protocol), for example, a signal for SDH, SONET, etc., and a signal for Ethernet (R) other than that].
As a result, a direct modulation signal, obtained from a high-speed signal which is handled in high-speed communication networks of a level of gigabits/sec, can be transmitted as it is. Conversely speaking, active control cannot be flexibly performed according to the content of information about an optical transmission signal, and it can also be said that the degree of freedom of the network is not high.
For instance, in a burst communication network where burst communication is performed, such as an Ethernet(R), etc., each of the optical signals (channels) of a WDM signal to be transmitted between the NEs 104 through the OADM node 103 does not always carry information. That is, there is an idle channel having no transmission data, although the channel itself is transmitted. Therefore, if such an idle channel can be utilized to transmit different information, then wavelength resources can be effectively utilized and immediate switching of communication paths, control for changing an add-drop structure, etc., can be flexibly performed according to an increase or decrease in traffic between nodes.
As described above, in the conventional OADM node 103, the wavelength adding-dropping function is realized by the optical switches 114 or rejection filter 127. Because of this, even if there is an idle channel, the channel cannot be utilized in an arbitrary OADM node 103 and the above-described active control cannot be realized. As a result, WDM transmission performance per one channel is not the maximum.
In addition, in the WDM transmission system employing OADM nodes 103, there are wavelengths whose OSNR is sufficient and wavelengths whose OSNR is not sufficient, depending on the number of nodes and the number of wavelengths. Therefore, in the case where a signal is added and dropped, the system becomes complicated, because it must be designed in consideration of the setting of a pass group.
For example, as shown in FIG. 21, in the case where it is assumed that optical signals whose wavelength is small are on a short-wavelength side, an optical signal of wavelength λ2 is passed through 2 spans. Because of this, the OSNR of the optical signal of wavelength λ2 becomes bad, compared with an optical signal of wavelength λ4 which passes through only one span. Such a phenomenon has to be taken into consideration over the entire network. In addition, the main function of the OADM node 103 is to increase or decrease the number of wavelengths to change the quantity of information to be transmitted when traffic is increased or decreased at a certain node 103. When the number of wavelengths is increased or decreased, control must be performed based on the correction information about the OSNR of the node 103 and about ASE light so that the communication state of the entire system becomes optimum.
Because of this, in the WDM transmission system of a ring structure or terminal-opposed structure in the existing WDM backbone, each of the transmitting and receiving systems in each node is provided with the spectrum analyzer 124 to acquire the level of light and the number of wavelengths for each span and optimize the level of light to be added and dropped.
However, if the optical switches 114 or rejection filter 127 is employed in the OADM node 103 to make a vacant wavelength, it looks as if an OSNR were enhanced within the node 103, because ASE light is removed in a narrow band. As a result, if the number of wavelengths is increased or decreased at a certain node 103, the correction information about the OSNR and ASE light will change and have influence on other nodes 103. Therefore, the number of wavelengths cannot be quickly and freely increased or decreased.
In addition, the above-described structure is nearly the same as a structure where two terminals are practically combined. At the same time, the optical signal monitoring system based on the OSC becomes complicated. As a result, even when one or two wavelengths are added and dropped, the expensive OADM node 103 with the spectrum analyzer 124 must be employed. Therefore, cost performance is extremely bad and the cost of the entire system becomes extremely high. In addition, as the size of the system becomes larger, the number of transmissions and receptions of the monitor control information between nodes (basically, information about the number of wavelengths), including preemphasis control, is increased. Therefore, an OSC of large capacity must be prepared and effective utilization of wavelength resources becomes difficult.
Furthermore, if the wavelength adding-dropping function is realized by the optical switches 114 or rejection filter 127, the intensity of light and number of wavelengths for each span are limited. Therefore, once services are carried out, the wavelength adding-dropping structure cannot be changed unless the services are stopped.